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Describe a the circle ACB about the triangle, and draw its diameter AE, and join EC: because the right angle BDA is equal b to the angle ECA in a semicircle, and the angle ABD to the B angle AEC in the same segmentc; the triangles ABD, AEC are equiangular; therefore, as d BA to AD, so is EA to AC; and consequently the rectangle BA, AC is equal to the rectangle EA, AD. If, therefore, from an angle, &c.
Q. E. D.
c 21. 3.
d 4. 6.
e 16. 6.
PROP. D. THEOR.
THE rectangle contained by the diagonals of a Sec N. quadrilateral inscribed in a circle is equal to both the rectangles contained by its opposite sides.
Let ABCD be any quadrilateral inscribed in a circle, and join AC, BD; the rectangle contained by AC, BD is equal to the two rectangles contained by AB, CD, and by AD, BC*.
Make the angle ABE equal to the angle DBC; add to each of these the common angle EBD, then the angle ABD is equal to the angle EBC: and the angle BDA is equal a to the an- a 21. 3. gle BCE, because they are in the same segment ; therefore the triangle ABD is equiangular B to the triangle BCE: wherefore bas
b 4. 6. BC is to CE so is BD to DA; and
C consequently the rectangle BC, AD is equal to the rectangle BD, CE: again, because the angle ABE is equal to the angle DBC, 'and the angle a BAE to the angle BDC, the tri
E angle ABE is equiangular to the triangle BCD: as therefore BA to AE,
D so is BD to DC; wherefore the rect
A angle BA, DC is equal to the rectangle BD, AE: but the rectangle BC, AD has been shown equal to the rectangle BD, CE; therefore the whole rectangle AC, BD d d 1. 2. is equal to the rectangle AB, DC, together with the rectangle AD, BC. Therefore, the rectangle, &c. Q. E. D.
c 16. 6.
* This is a lemma of Cl. Ptolomæus, in page 9. of his xeyaan OuYtagus.
ELEMENTS OF EUCLID.
when it makes right angles with every straight line meeting it
in one of the planes perpendicularly to the common section of
contained by that straight line and another drawn from the
by two straight lines drawn from any the same point of their
Book XI. Two planes are said to have the same, or a like inclination to one
another, which two other planes have, when the said angles of inclination are equal to one another.
VIII. Parallel planes are such which do not meet one another though produced.
IX. A solid angle is that which is made by the meeting of more than See N.
two plane angles, which are not in the same plane, in one point.
X. " The tenth definition is omitted for reasons given in the notes.' See N. XI.
See N. Similar solid figures are such as have all their solid angles equal,
each to each, and which are contained by the same namber of
tuted betwixt one plane and one point above it in which they
that are opposite are equal, similar, and parallel to one ano-
XIV. A sphere is a solid figure described by the revolution of a semicircle about its diameter, which remains unmoved.
through the centre, and is terminated both ways by the super-
XVIII. . A cone is a solid figure described by the revolution of a right
angled triangle about one of the sides containing the right
angle, which side remains fixed. If the fixed side be equal to the other side containing the right
angle, the cone is called a right angled cone ; if it be less than the other side, an obtuse angled, and if greater, an acute ang cone.
XIX. The axis of a cone is the fixed straight line about which the tri. angle revolves.
angled parallelogram about one of its sides, which remains
XXII. The axis of a cylinder is the fixed straight line about which the parallelogram revolves.
XXIII. The bases of a cylinder are the circles described by the two revolving opposite sides of the parallelogram.
XXVII. An octahedron is a solid figure contained by eight equal and equilateral triangles.
XXVIII. A dodecahedron is a solid figure contained by twelve equal pentagons which are equilateral and equiangular.
figures, whereof every opposite two are parallel.
PROP. I. THEOR.
ONE part of a straight line cannot be in a plane See N. and another part above it.
If it be possible, let AB, part of the straight line ABC, be in the plane, and the part BC above it : and since the straight line AB is in the plane, it can be produced in that plane : let it be pro
С duced to D: and let any plane pass through the straight line AD, and
A be turned about it until it pass
D through the point C; and because the points B, C are in this plane, the straight line BC is in it a : therefore there are two a 7. def.i. straight lines ABC, ABD in the same plane that have a common segment AB, which is impossible b. Therefore, one part, &c. b Cor.11. Q. E. D.
PROP. II. THEOR.
TWO straight lines which cut one another are in See N one plane, and three straight lines which meet one another are in one plane.
Let two straight lines AB, CD cut one another in E; AB, CD are one plane: and three straight lines EC, CB, BE which meet one another, are in one plane.
Let any plane pass through the straight A D line EB, and let the plane be turned about EB, produced, if necessary, until it pass through the point C: then because the points E, C are in this plane, the straight
E line EC is in it a : for the same reason, the
a 7. def.1. straight line BC is in the same ; and, by the hypothesis, EB is in it: therefore the three straight lines EC, CB, BE are in one plane : but in the plane in which EC, EB CA
B are, in the same are b CD, AB: therefore,
b 1. 11 AB, CD are in one plane. Wherefore, two straight lines, &c. Q. E. D.